Clinical x-ray imaging has always been based on the biological tissue's differences in x-ray attenuation ever since Roentgen discovered x-ray over 100 years ago. However x-ray-tissue interaction causes x-ray phase changes as well. We have identified the four clinically important factors that affect the x-ray phase visibility in clinical imaging. These factors are: body part attenuation, the spatial coherence of incident x-rays from an x-ray tube, the polychromatic nature of the x-ray source, and radiation dose to patients for clinical applications. A Wigner-distribution-based theory of phase-contrast imaging is presented to account for the effects of these four factors. Numerical simulations for x-ray phase-contrast mammography for phase reconstruction are described. © 2005 IEEE.